Genus Characteristics

Lawsonia intracellularis, a slender, curved Gram-negative rod, is classified in the delta subdivision of the Proteobacteria. The full genome has been sequenced and is available on the EMBL-EBI website (www.ebi.ac.uk/embl/Contact/collaboation.html accessed 21 December 2012). It is most closely related to Bilophila wadsworthia and the sulphate-reducing Desulphovibrio species. Lawsonia intracellularis is an obligate intracellular pathogen and causes disease primarily in pigs. However, it is increasingly recognized as a cause of diarrhoea in foals and disease has been recorded in a number of other species including hamsters, deer and ostriches. Strains isolated from different species appear to be closely related (Cooper et al. 1997). The organism can be grown only in enterocyte cell lines and is microaerophilic.

Natural Habitat

Lawsonia intracellularis grows intracellularly in enterocytes, particularly those of the terminal ileum, in pigs and other species. It can survive in the environment for periods of approximately two weeks.

Pathogenesis and Pathogenicity

The characteristic lesion associated with infection is a proliferative enteropathy but clinical disease can range from an acute haemorrhagic syndrome to a subclinical condition associated with poor production. In pigs, all ages are susceptible to infection but under farm conditions, infection appears to occur approximately six weeks after weaning, with shedding of the organism persisting for between two and six weeks (Stege et al. 2004). Interaction with other microorganisms of the intestinal flora appears to be essential for the development of clinical disease as neither infection nor colonization results from the exposure of gnotobiotic pigs to pure cultures of L. intracellularis. In addition, recent studies indicate that active proliferation and differentiation of crypt cells, as occurs at weaning, as well as infection and multiplication of Lawsonia organisms, is necessary for the production of the characteristic proliferative lesions (McOrist et al. 2006).

The pathogenesis of proliferative enteropathy has been reviewed by Smith & Lawson (2001). The principal cells infected are the epithelial cells lining the intestinal crypts. Exact mechanisms involved in colonization and invasion of enterocytes have not been identified although a surface antigen apparently involved in attachment to and entry into epithelial cells of the intestine has been documented (McCluskey et al. 2002). The cell membrane remains intact as the organism enters the cells but the organisms do not remain in the vacuole formed but multiply free in the cytoplasm and are located principally in the apical area of the cell. Following infection, the cells begin to proliferate and migrate to populate the epithelium. In more severe disease, the organism may be found in tissues other than the epithelium, being found within macrophages in the lamina propria, submucosa and tonsils and in intestinal capillaries and lymphatics (Love & Love 1979, Jensen et al. 2000). It is unclear whether differences in disease manifestation reflect differences in virulence of bacterial strains or alterations in the host immune response. It is suggested that the organism may be able to modulate the immune response as the inflammatory response is notably limited, particularly in the early stages of disease (Rowland & Rowntree 1972).

Laboratory Diagnosis

Lawsonia intracellularis can be grown only in enterocyte cell lines and few laboratories worldwide have the expertise required. Thus, isolation of the organism from clinical specimens is not generally available as a method of diagnosis. Vannucci et al. (2012) recently described an alternative method for culture of Lawsonia organisms using an Original Space Bag (Storage Packs, San Diego, CA). This method allows culture without the need for a tri-gas incubator and thus may be feasible for a greater number of laboratories than the original methods described.

In the past, presumptive diagnosis of infection with L. intracellularis was based on history, clinical signs and the demonstration of typical gross and histopathological lesions following postmortem examination. Silver staining of histopathological sections has been used to reveal numerous curved intracellular organisms in the epithelial lining of the ileum, but the sensitivity of this technique was poor. Immunohistochemical techniques were developed and are still used as sensitive and specific diagnostic methods (Jensen et al. 2010).

In recent years considerable progress has been achieved in developing diagnostic tests suitable for use in the live animal, although the most accurate results still require the demonstration of the organism in intestinal tissues collected from animals postmortem. Screening of herds for infection can be carried out using either serological tests or by detection of the organism in faecal samples. The serological tests used include the indirect fluorescent antibody test, an immunoperoxidase monolayer assay and ELISA techniques; some of these tests are available commercially. Detection of the organism in faecal samples can be done using PCR, including ‘real-time’ PCR (Lindecrona et al. 2002). Multiplex PCR techniques for the detection of Salmonella species, Brachyspira hyodysenteriae and L. intracellularis (Elder et al. 1997, Suh & Song 2005) as well as Brachyspira hyodysenteriae. Brachyspira piloscoli and L. intracellularis (La et al. 2006) have been described. Jacobson et al. (2004) evaluated serology, PCR techniques and postmortem examination for diagnosis of L. intracellularis infection and concluded that serology or repeated faecal sampling for PCR, or both, should be used to determine the presence of the microorganism in a herd. Lawsonia intracellularis can also be demonstrated in infected faeces using techniques such as immunomagnetic separation and ATP bioluminescence (Watarai et al. 2005) but such techniques are not widely available in laboratories.